[media] docs-rst: parse-headers.pl: use the C domain for cross-references

Instead of keep using the normal reference, move to the C
domain ones. Using C domains everywhere will allow
cross-references between kAPI and uAPI docs.

Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>